1. Technical Field
The field of the present invention is wireless communication devices and methods for using the same. In particular, the present invention relates to Code Division Multiple Access (CDMA) spread spectrum transmission devices and methods.
2. Background Art
Code Division Multiple Access (CDMA) is a communications technology based on the principals of spread spectrum communication. Users in a CDMA system share the same carrier frequency and bandwidth, but are differentiated by encoding data with a pseudo-noise (PN) spreading signal. Since users share frequency and bandwidth, CDMA allows more users to access the wireless network simultaneously than with other systems such as Time Division Multiple Access.
Generally, a CDMA communication system is deployed using a cellular-type network system. In a cellular system the geography is divided into several adjacent zones of coverage, called "cells". Each cell in the network has at least one base station transceiver that establishes a communication link with subscriber equipment located within or near the cell. Subscriber equipment is often a mobile unit, as exemplified by car telephones. As the subscriber equipment moves from one cell to an adjacent cell, the system provides a "handoff" between base stations for any communication in progress. Thereby, the subscriber moves from cell to cell without interruption to the communication.
CDMA is presently gaining widespread acceptance in the United States and other countries throughout the world. In the United States, CDMA has matured into standards that are administered by the Telecommunications Industry Association (TIA). The present standards are known as IS-95 (TIA) and J-STD-008 (ANSI).
Present devices using CDMA technology as defined by IS-95 utilize a base clock rate of 1.2288 MHz. This base clock rate is generally referred to as the "chip" rate, and is the rate at which the system encodes and spreads the communication signal. IS-95 compliant systems are 2.sup.nd generation CDMA systems that are generally referred to as 1X systems. The "1X" refers to the chip rate of 1.2288 Mhz. With a chip rate of 1.2288 MHz, an IS-95 compliant system occupies a spectral bandwidth of 1.25 MHz.
1X CDMA as defined by current standards IS-95 is a "Frame-Based" communication system that derives its timing reference for system operation from the Global-Positioning-System (GPS). Transmissions are referenced to a common system-wide timing reference provided by the GPS system. This time reference is referred to as the Universal-Coordinated-Time (UTC). The UTC provides a signal that CDMA equipment receives and decodes to establish a system-wide time base. The start of CDMA system time is defined to be Jan. 6.sup.th, 1980 00:00:00 UTC, which coincides with the start of GPS time. From 00:00:00 UTC, 1X CDMA codes were initialized.
For 1X CDMA the spreading code contains 2.sup.15 (32,768) codes. The spreading code, sometimes referred to as the Short-Code-Pseudo-Noise (PN) sequence, is utilized at the chip rate of 1.2288 MHz. Therefore, the PN spreading code has a period of 26.67E-3 seconds (32,768 codes/1.2288 MHz).
Also, 1X CDMA uses a second code to scramble transmission to and from the subscriber unit. The second code is called the long code. The long-code-PN-sequence is 2.sup.41 chips in length, also encoded at a rate of 1.2288 MHz. Therefore, the long-code-PN-sequence has a period of about 20.71 days (2.sup.41 /1.2288E6/60/60/24). The long-code-PN-sequence, like the short code, is also initialized from 00:00:00 UTC.
Referring now to FIG. 4 the frame timing of a CDMA system is diagramatically shown. The diagram shows a first even second tick mark 20 received from the GPS system and a second even second tick mark 22 received from the GPS. The elapsed time between tick mark 20 and second tick mark 22 is therefore two seconds. This two second elapsed time is divided evenly into 25 synchronization frames 24. Each of the sync frames 24, such as sync frame 26, is therefore 80 msec long. As previously discussed, the PN short code in a 1X system repeats every 26 2/3 msec. As the 26 2/3 msec period is exactly one-third of the 80 msec sync frame, the PN spreading code is repeated three times within the 80 msec sync frame. Thereby FIG. 4 shows repeat 0 (28) of the PN code, repeat 1 (29) of the PN code, and repeat 2 (30) of the PN code are all contained within the sync frame 26.
Each time the PN spreading repeats, the PN spreading code is said to "roll". Therefore, PN roll 31 corresponds to the beginning of the repeat 0 (28), PN roll 32 corresponds to the beginning of the PN repeat 1 (29), and PN roll 33 corresponds the beginning of the repeat 2 (30). Therefore, in a 1X CDMA system as shown in FIG. 4, each sync superframe 26 will comprise three PN rolls: 31, 32 and 33.
In the current CDMA technology, identifying the beginning of a sync channel message capsule amounts to several tasks:
1. Read the SOM bit (Start Of Message) located at the 26 2/3 frame boundary; PA1 2. If this value is One, read the sync channel message capsule, decode its CRC parity pattern; and PA1 3. If parity is zero, the beginning of the sync message is identified, else, go to step 1.
Thus, the process involved in identifying the sync message is a complex operation that may have to be repeated multiple times before a positive identification occurs. The repeated PN Roll introduces an overhead due to the ambiguity in the location of the beginning of the message. More specifically, each PN Roll may appear to be the beginning of the sync channel super frame. This process described above is carried out until a positive identification is achieved. This identification process is carried out repeatedly while the subscriber unit is in a sync channel acquisition mode. This timing ambiguity causes delay in synchronization, and causes the subscriber equipment to use more power. If the subscriber equipment is battery operated, batteries deplete faster or must be made larger to accommodate the power drain.
U.S. Pat. No. 5,703,873 describes a method and apparatus for synchronizing subscriber equipment with base stations in a CDMA radio network. This patent describes a modification to the existing CDMA standard wherein the pilot channel is modulated to contain information that allows a subscriber unit to more efficiently synchronize with the base station. However, such a system fails to be compatible with existing implementations of the CDMA communications system. Without backward compatibility, existing investments in base stations and subscriber units may be lost.
Due to the popularity of the CDMA 1X system currently deployed, CDMA systems are becoming saturated with users and are unable to satisfy the communication demands of progressing technologies. Therefore, it is necessary that a new CDMA system be developed for handling increased data capacities. Such increased capacities allow users to transmit data at a higher rate and more efficiently. This next generation of CDMA system, thereby will hereinafter be referred to as a third generation (3G) CDMA system.
It is generally recognized that two options for 3G CDMA exist. As shown in FIG. 1, the first option is a multiple-carrier implementation 42. The multiple-carrier implantation 42 uses parallel implementations of the current 1X CDMA systems, thereby allowing for larger bandwidth by processing more than one 1.25 MHz. The second option is a direct-sequence implementation that increases the bandwidth and defines a new system that increases the system chip rate by multiples of 1.2288 Mhz.
Referring to FIG. 2, in the multi-carrier CDMA system 42, the processing unit 46 is a bank of parallel processors, 47, 48, and 49, one for each of the RF carriers. Although conceptually simple to implement, the multi-carrier system vastly increases the part-count and complexity of the CDMA system. In contrast, as shown in FIG. 3, the processing unit 48 for a direct sequence implementation retains the simpler architecture of an 1X system. While each of the above options occupy a similar bandwidth, clearly the Direct Sequence CDMA has an advantage over the latter in terms of component count. Unfortunately, the direct-sequence implementation aggravates the timing ambiguities described above. For example, if the CDMA system operates at a clock rate 3 times the base chip rate, then the PN code will repeat 9 times in each synchronization frame. Now, rather than 3 PN rolls as in a 1X system, the system will need to contend with 9 PN rolls.
Therefore, there exists a need for a new 3G CDMA system with increased data capacity but backwardly compatible to existing CDMA implementation.